/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.iotdb.db.engine.storagegroup;

import org.apache.iotdb.commons.path.PartialPath;
import org.apache.iotdb.db.engine.querycontext.QueryDataSource;
import org.apache.iotdb.db.exception.query.QueryProcessException;
import org.apache.iotdb.db.query.context.QueryContext;
import org.apache.iotdb.tsfile.read.filter.basic.Filter;

import java.util.Collections;
import java.util.List;

/**
 * It's a virtual data region used for query which contains time series that don't belong to any
 * data region.
 */
public class VirtualDataRegion implements IDataRegionForQuery {

    private static final String VIRTUAL_DB_NAME = "root.__virtual";

    private static final QueryDataSource EMPTY_QUERY_DATA_SOURCE =
            new QueryDataSource(Collections.emptyList(), Collections.emptyList());

    public static VirtualDataRegion getInstance() {
        return VirtualDataRegion.InstanceHolder.INSTANCE;
    }

    @Override
    public void readLock() {
        // do nothing, because itself is thread-safe already
    }

    @Override
    public void readUnlock() {
        // do nothing, because itself is thread-safe already
    }

    @Override
    public QueryDataSource query(
            List<PartialPath> pathList, String singleDeviceId, QueryContext context, Filter timeFilter)
            throws QueryProcessException {
        return EMPTY_QUERY_DATA_SOURCE;
    }

    @Override
    public long getDataTTL() {
        return Long.MAX_VALUE;
    }

    @Override
    public String getDatabaseName() {
        return VIRTUAL_DB_NAME;
    }

    private static class InstanceHolder {

        private InstanceHolder() {
        }

        private static final VirtualDataRegion INSTANCE = new VirtualDataRegion();
    }
}
